[sdiy] Micro as a Linear to Exponential converter?

[Antti Huovilainen]

On Fri, 14 Aug 2009, Tom Wiltshire wrote:

> I'd just like to agree with Colin's outlined scheme above. I've done similar 
> things. Antti's comment that "the code part is trivial" is a bit misleading, 
> IMO.

I meant that when you have the generic MCU thing working, the exponential 
conversion (read: interpolated table lookup) itself truly is trivial. You 
can just write it all in C. MCUs are plenty fast nowadays.

Example code below:

unsigned short noteTable[128];

// note = 0..126, frac = 0..65535 (equivalent to 0..0.99999)
unsigned short expo(short note, unsigned short frac)
{
   long  l1, l2, f, y;
   f = frac;
   l1 = noteTable[note];
   l2 = noteTable[note+1];
   y = l1 * 65536L + (l2-l1)*frac;
   return (unsigned short) (y >> 16);
}

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova